Filters are known to provide attenuation of signals having frequencies outside of a particular frequency range and little attenuation to signals having frequencies within the particular range of interest. As is also known, these filters may be fabricated from ceramic materials having one or more resonators formed therein. A ceramic filter may be constructed to provide a lowpass filter, a bandpass filter, or a highpass filter, for example.
Ceramic filters typically employ quarter-wavelength type resonators with one end electrically open and the other end shorted to ground in combline like design. This design offers compact size and rugged construction in a slim, low-profile component. Moreover, this design offers transmission zeros between pairs of resonators and only requires a printed pattern on one surface of the filter block.
FIG. 1 shows a ceramic filter with a combline design, representative of the prior art. A filter 100 is provided which has resonators 102. The resonators 102 are said to have a combline design because they are both open-circuited at one end 104 and short-circuited at the other end 106. FIG. 2 shows a schematic of the electrical circuit that corresponds with filter 100. Referring to FIG. 2, resonators 102 are provided. The combline coupling 108 is shown inside the dashed-line box. In a combline filter design, the inter-resonator coupling is described as a series connected short circuited stub. Combline coupling is well known in the art.
One disadvantage of the traditional combline design, however, is the fact that these block filters oftentimes require an external metallic shield attached to the open-circuited end of the block in order to minimize the parasitic coupling between non-adjacent resonators and to achieve acceptable stopbands and satisfactory harmonic performance. Filter designers expand much effort in designing a shield which is compatible with the block design and is easily manufacturable and attachable. Such shields are typically stamped from sheet metal and attached by a soldering operation.
Another design alternative involves the use of an interdigital resonator design in the dielectric block of ceramic. Interdigital resonator designs allow full quarter-wavelength designs which provide higher electrical Q, an important property related to loss. Whereas a block filter with an interdigital resonator design does not typically require a metallic shield, it does create other design challenges. For example, the strong inter-resonator coupling associated with this design necessitates large spacings between the resonators for narrow-band filter designs. This may lead to filters which are undesirably large in volume. Additionally, the transmission zeros between the pair of resonators, which is found in the combline filter design, is not found in the interdigital design. Transmission zeros are important to obtain a highly selective frequency response in the compact filters needed for modern communication equipment.
FIG. 3 shows a ceramic filter with an interdigital design, representative of the prior art. A filter 300 is provided which has resonators 302. The resonators 302 are said to have an interdigital design because they are each open-circuited at opposite ends 304 of the filter block. Similarly, the resonators are each short-circuited at opposite ends 306 of the filter block, creating the interdigital design. FIG. 4 shows a schematic of the electrical circuit that corresponds with filter 300. Referring to FIG. 4, resonators 302 are provided. The interdigital coupling 308 is shown inside the dashed-line box. In an interdigital filter design, the inter-resonator coupling is described by a series transmission line. Interdigital coupling is also well known in the art.
A ceramic filter design which exploited the best attributes of both interdigital filter designs and combline filter designs to provide a filter which has transmission zeros, superior stopband rejection, and good harmonic performance in a small, compact, low-profile package that did not require an external metal shield would be considered an improvement in the art.